1. Field of the Invention
The present invention relates to antennas, and, more specifically, to a small antenna particularly suitable for portable radio equipment, and having a radiator of meander line shape.
2. Description of the Related Art
As portable radio equipment has become miniature and light-weight in recent times, there has also been significant development in small antennas suitable for use in such equipment. Any such small antenna should be convenient and simple for a user to operate, and should have an omnidirectional antenna pattern in azimuth and a relatively high gain in the elevation. In addition, when the portable equipment is placed near a human body, the presence of the human body should minimally affect the basic characteristic of the antenna, that is, input impedance and gain variation.
One solution to meet the above requirements is disclosed in U.S. Pat. No. 4,700,194 to Ogawa et al, issued Oct. 13, 1987. According to the above patent, if the antenna current flows on a ground circuit and on the equipment terminal case, the current flowing on the antenna is varied if the terminal case is placed in the vicinity of the human body, so that the input impedance and the gain of the antenna may be further varied. As a result, even without using a quarter-wave trap or a balance to unbalance transformer (hereinafter, referred to as balun) as used in prior art sleeve antennas, good electrical isolation may be provided between the antenna and the ground circuit of a coaxial transmission line or of the electric circuit.
FIGS. 1A and 1B are diagrams showing the construction of a prior art quarter-wavelength microstrip antenna (hereinafter, referred to as QMSA) which is described in the above U.S. Pat. No. 4,700,194. In FIG. 1B, centering around a dielectric 61, the antenna includes a radiation element on one surface of the dielectric and a ground element on another surface. A first feed radiation element 62 (first feeding means) is electrically connected to a signal line of the transmission line. A second feed radiation element is constructed on the ground element so as to electrically connect the ground line of the transmission line and the ground element, which is located at a position where the voltage of the standing voltage wave induced on the ground element becomes minimum. Now, in a conventional microstrip antenna, the ground plane no longer acts as the ground if the size of the ground plane is small relative to the wavelength of the operating frequency. In this case, a sinusoidal variation of a voltage distribution, or a voltage standing wave is induced on the ground plane. As a result, a parasitic current is induced on the outer conductor of the coaxial transmission line. In the antenna of FIGS. 1A and 1B, to reduce the generation of such parasitic current to a minimum, the outer conductor of the transmission line is connected to the ground element at a second feed point where the voltage of the standing voltage wave induced on the ground element becomes minimum. With this structure, the parasitic current on the transmission line can be reduced or eliminated without any quarter-wave trap which is used in conventional sleeve antenna configurations. Accordingly, the variation of the antenna characteristics can be considerably reduced in the event that the antenna is placed in the vicinity of the human body or an electric circuit.
FIGS. 2 and 4 are diagrams showing variation of the gain characteristic depending upon lengths L, Gz of a quarter-wavelength microstrip antenna according embodiments of the prior art, and FIG. 3 is a diagram showing variation of the gain characteristic depending upon width W of a quarter-wavelength microstrip antenna according an embodiment of the prior art.
One disadvantage of the prior art quarter-wavelength microstrip antenna is that variation of the efficiency characteristic of the antenna depends considerably on the thickness of the printed circuit substrate (hereinafter, referred to as PCB). That is, the antenna gain is related to the thickness of the PCB. A thicker PCB results in higher gain, but increases the size and weight of the antenna, thereby causing inconvenience to the user as it is more difficult to carry. To the contrary, if the PCB is thin, while the antenna can be easily carried by a user, the gain of the antenna may be consequently diminished.